Fix #18863: Add extensible ClientSettings to ClientRegistration

[pranavmanglik]

Description

This pull request introduces an extensible ClientSettings configuration within ClientRegistration. Currently, ClientRegistration handles core OAuth2 properties, but adding provider-specific or advanced settings (such as custom PKCE requirements or extended client metadata) often requires modifying the core class or maintaining complex external maps.

By encapsulating these configurations in a dedicated, immutable ClientSettings object, we provide a cleaner API for future extensions without bloating the ClientRegistration root. This aligns with the framework's goals of modularity and extensibility for modern OAuth2/OIDC flows.

Key Changes

• Introduced ClientSettings: A new configuration class held by ClientRegistration to store specialized client behavior settings.
• Updated ClientRegistration.Builder: Added the .clientSettings(ClientSettings) method to the builder pattern to allow for fluent configuration.
• Enhanced Validation: Integrated logic within ClientRegistration.Builder to ensure that provided ClientSettings (e.g., requireProofKey) are compatible with the selected AuthorizationGrantType.
• Testing: Added comprehensive unit tests in ClientRegistrationTests covering valid configurations, edge cases, and validation failures.

Related Issues

Ref: #18863 Checklist

• Fixed checkstyle issues using ./gradlew checkstyleMain
• Added unit tests in ClientRegistrationTests
• All tests passed via ./gradlew :spring-security-oauth2-client:test
• Documentation updated